This research will examine membrane properties of identified, isolated bipolar cells associated with the OFF-center pathway of the retina. Serotonin immunoreactivity will be used to identify isolated bipolar cells studied using the whole-cell version of the patch-clamp recording technique. Both ligand- and voltage-gated channels will be examined. Immunocytochemical evidence from the skate retina has shown that these cells possess the morphological characteristics of OFF-center bipolar cells and this evidence will be further tested through histo- pharmacological manipulations. Difficulties in recording bipolar cells in the intact retina and in identifying them in isolated cell preparations have impeded progress in understanding the mechanisms of this critical link in the visual pathway and its importance to retinal disorders. Recently, this difficulty has been overcome in studies of ON-center rod bipolar cells by using protein kinase C immunoreactivity to identify dissociated cells used for patch- clamp studies. The experiments described here, therefore, offer the opportunity to examine a second class of identified bipolar cell in isolation and should provide information relevant to our understanding of OFF-center interplexiform pathways. Since the skate has an all-rod retina, findings will be particularly pertinent to revealing the role of rod input to OFF pathways. Protein kinase C-immunoreactive bipolar cells typical of the ON-center rod bipolar cells of other retinas have already been demonstrated in skate so comparison with findings from current studies on such cells can reasonably be made. Bipolar cells are the afferent neurons by which all visual information is conveyed from the outer to the inner plexiform layer of the vertebrate retina. In addition, it is at this level that separate ON and OFF pathways are first represented. A detailed knowledge of the properties of identified cells from these pathways will answer important questions as to how this organization contributes to the processing of visual information in vertebrates.